This invention relates to radomes, and, more particularly, to the fabrication of ceramic radomes.
Many missiles and aircraft carry onboard radar sets for surveillance or targeting purposes. The radar set includes a transceiving antenna that is mounted to the airframe and points outwardly. The antenna is covered with a protective structure, generally termed a radome, to protect it against aerodynamic forces and against damage by foreign objects encountered during flight.
The radome must have sufficient structural strength and fracture resistance to withstand the aerodynamic forces and foreign object impingement, and it should offer minimal aerodynamic resistance. The radome also must cause minimal attenuation and distortion of the outgoing and incoming radar energy. These requirements are readily met by conventional nonmetallic materials for radomes used on aircraft flying at relatively low speeds.
When the radome protects a forward-facing radar antenna of a high-speed missile or aircraft, such as one which flies faster than several times the speed of sound in the atmosphere, the radome is heated during service to high temperatures by aerothermal friction. It is therefore necessary to fabricate and employ a ceramic radome.
In one fabrication approach, large ceramic radomes are conventionally fabricated by forming a mixture of a binder and a mass of ceramic particles, casting this "slip" to shape in a mold, drying the slip, vaporizing the binder, and sintering the ceramic particles. The drying operation requires a long period of time wherein the water in the slip diffuses out of the molded article, through a porous wall mold. Water soluble species in the molded article migrate with the water, producing inhomogeneities and density gradients in the material. The resulting dried "green" body has very little strength and is quite fragile, and is therefore easily damaged in the subsequent process steps. One characteristic of the fabrication of large ceramic articles by such slip casting is that the attainment of precise configurations and dimensions is difficult due to excessively large shrinkage and the consequent shape changes and warping that occur during the vaporizing and sintering steps. The resulting distortion is somewhat uncontrollable and often leads to shape variations in the final article that are unacceptable because they adversely affect the aerodynamics of the missile or aircraft. The cost of production of ceramic radomes is therefore high due to a relatively low yield and the cost of machining and reworking to overcome the distortions that occur.
In another fabrication approach for producing a Pyroceram.RTM. glass-ceramic material, molten silicate liquid is cast into a mold. The liquid solidifies as a glass. The glass is thermally treated to form a crystalline phase within the glass matrix, resulting in a glass-ceramic material. Because of the nature of the process, the cast radome precursor is typically twice as thick as desired, requiring extensive subsequent machining to reduce the thickness of the blank to that of the final radome.
There is a need for an improved approach to fabricating ceramic radomes that has a greater yield of acceptable articles. The present approach fulfills that need, and further provides related advantages.